The propulsion system of a marine vessel comprises a power source (usually a diesel engine, possibly with an appropriate transmission) that is located inside the engine room within the vessel's hull, and a propeller located at the aft of the vessel outside the hull. The propeller is by a propeller shaft. The propeller shaft is taken inside the hull of the marine vessel by means of a stern tube. The inside of the stern tube is provided with bearing means for carrying the weight of both the propeller shaft and the propeller. The stern tube is fastened to the hull by means of a stern frame. The aft end of the stern tube i.e. the end nearest the propeller, is provided with a sealing assembly the purpose of which is to prevent the sea water from entering the hull of the marine vessel and the bearing lubricating oil from leaking to the sea. However, the sealing members wear or deteriorate over time whereafter they allow either sea water or oil or both pass the sealing. In itself the sealing members do not reveal any problem but, unless any specific measures are taken, the seal failure may only be observed visibly either as oil in the sea or as sea water in the bilge. At first the leakage is most often so minimal that it does not cause any problems in the operation of the sealing or propulsion, but over time the deterioration of the sealing members grow, and the leakage is so voluminous that quick actions are required. Thus it is of utmost importance to detect a leak in the sealing assembly as soon as possible i.e. immediately after its occurrence.
There are a few prior art patent documents, i.e. U.S. Pat. No. 4,174,672, and U.S. Pat. No. 4,483,540 that discuss the above mentioned problem. Both documents teach that the propeller shaft sealing assembly comprises a series of annular seals such that there is one set of seals for the sea water, and one set for the oil. For instance U.S. Pat. No. 4,174,672 teaches that there is one seal ring in the sea water seal set, and two seal rings in the oil seal set. Between these sets of seals there is a void space into which either sea water or oil leaks in case of seal failure. Both prior art patent documents teach how the above mentioned void space is connected by means of a monitor line arranged in connection with the sealing assembly and the stern tube or frame to a specific tank that is continuously monitored. In other words, as soon as leakage occurs the leaked liquid flows along the monitor line into a leakage fluid tank and the leakage is detected well before any real damage occurs. The leakage fluid tank may be monitored, just to name a few alternatives, either by means of a sight glass, or by electrical means monitoring the fluid level in the tank and giving a visible and/or audible warning when a change in the fluid level takes place. In this manner the warning is received in an early stage, and the maintenance of the sealing assembly need not be done immediately, but possibly only during normal maintenance routines.
The monitor line of the leakage fluid is nowadays made to run through several casing rings and a casing flange that form part of the sealing assembly. A part, in practice most, of the casing rings are used for housing annular seals, and a part, normally only the casing flange, for attaching the sealing assembly to the stern tube or frame. The casing rings and the casing flange are attached to each other by means of series of circumferentially arranged bolts. A practical problem exists, as, when designing the stern tubes or frames, the shipyards normally, for a certain stern tube or frame diameter, use a certain number of attaching bolts for fastening the seal assembly to the stern tube or frame, whereby the location of the holes for the bolts is, in a way, standard. However, when positioning the hole, or holes, as there may be several holes, for the leakage fluid monitor line the shipyards seemingly do not apply any standard, but the monitor line is positioned more or less at random. The only rule seems to be, when looking from historical perspective, that the hole/s is/are arranged at a side of the centreline running along a vertical diameter of the stern tube such that the hole/s is/are positioned about 10 to 35 degrees from the centreline.
Now that the sealing assemblies forming the seal between the stern tube or frame and the propeller shaft are often delivered with the propulsion system or by some other supplier the sealing assembly supplier has to make the sealing assembly match to the aft end mounting face of the stern tube or frame. Each, even small change in the angular positioning of the monitor line in the stern tube or frame has resulted in that the seal assembly supplier has to redesign the entire sealing assembly. In practice, if the change in the angular position of the monitor line has been less than 0.5 degrees, the monitor lines in the sealing assembly and in the stern tube or frame have overlapped such that a new design of the sealing assembly has not been needed, but outside the 0.5 degree tolerance design has been a rule. The above described practice results in a huge number of sealing assembly components having slightly differing, dimensions.
An object of the present invention is to provide a propeller shaft sealing arrangement in which at least some problems of the prior art are minimized.
It is also an object of the present invention to provide an arrangement for sealing the propeller shaft of a marine vessel where the positioning of the monitor line for the fluid leaking in case of seal failure has been optimized.
It is a further object of the present invention to provide a sealing assembly, which takes into account possible changes in the angular/circumferential location of the monitor line in the stern tube or frame such that the same sealing assembly components may be used as often as possible irrespective of said changes.
It is a further object of the present invention to provide a casing flange by means of which it is possible to use standard seal casings in spite of changes in the angular/circumferential location of the monitor line in the stern tube or frame such that the same sealing assembly components may be used as often as possible irrespective of said changes.
It is a further object of the present invention to provide a spacer by means of which it is possible to use standard seal casings in spite of changes in the angular/circumferential location of the monitor line in the stern tube or frame such that the same sealing assembly components may be used as often as possible irrespective of said changes.